Recently, there have been found superconductive ceramics that can be used at liquid nitrogen temperature, exerting great industrial impact.
However, to use these superconductive ceramics in practical applications, it is essential to form these materials into wires, and it has been an important technical task how to form such hard, brittle ceramics into wires.
On the production of a wire material comprising a superconductive ceramic, the BULLETIN OF CERAMIC SOCIETY OF JAPAN, volume 22 (1987) No. 6, on page 526, describes a production method for a wire material, in which a ceramic powder is charged into a silver pipe which, after being drawn into a wire and wound into a coil, is heat-treated in an oxygen atmosphere. In addition, the Algonne National Institute has proposed a production method for a wire material comprising a superconductive ceramic, in which a Y-Ba-Cu-O powder is mixed with a polymer and formed into a wire, which is burnt to yield a flexible wire.
However, the former method, in which the ceramic powder is heat-treated in the interior of the metal pipe, is difficult to yield a dense sinter and achieve a sufficient superconductive contact between particles. Moreover, the method is defective in that the metal tends to diffuse into the superconductive ceramic during the heat treatment and it is difficult to precisely control the oxygen content which has substantial effects on characteristics of the resulting superconductive wire. The latter method is defective, among others, in that it is difficult to maintain the shape of the material during the combustion of the polymer until sintering begins, and that dense sintering causes growth of grains resulting in deteriorated mechanical characteristics and even causes decomposition which results in considerably lowered superconductive characteristics. Further, in Japanese Journal of Applied Physics, vol. 26 (1987) Supplement 26-3, on page 1211, is disclosed a production method for a wire in which a solution containing nitrates of Y, Ba, and Cu is mixed with powders of Y.sub.2 O.sub.3, BaCO.sub.3, and CuO, and the mixture is formed into a wire and then fired. However, this method is also defective, among others, in that dense sintering causes growth of grains, resulting in deteriorated mechanical characteristics.
As described above, in the prior art procedures, there have been known no production methods for wires consisting of densely sintered fine crystal grains of superconductive ceramics which have a high critical temperature and a high critical current density.
With a view to obviate the prior art defects of superconductive ceramic wire materials, it is a primary object of the present invention to provide a superconductive ceramic wire material which comprises densely sintered fine crystals and has a high critical temperature and a high critical current density, and a method for producing the same.